This invention relates generally to medication infusion pumps particularly of the type designed for implantation directly into the body of a patient and for programmed operation to deliver medication to the patient. More specifically, this invention relates to an improved implantable infusion pump having a simplified fluid pressure reservoir charged with a selected pressure fluid in liquid-gas phase for maintaining a supply of a selected medication under controlled pressure conditions. The improved pressure reservoir incorporates internal spacer means for preventing complete phase change of the selected pressure fluid to a liquid state.
Medication infusion pumps are generally known in the art for use in delivering a selected medication to a patient in a scheduled or preprogrammed manner. In recent years, such infusion pumps have been developed in compact form adapted for direct implantation into the body of a patient, and to deliver a specific medication such as insulin to the patient in discrete doses over an extended period of time. An implanted infusion pump of this general type includes an internal medication chamber for receiving and storing a supply of the selected medication in liquid form, with the medication being subjected to a predetermined storage pressure to insure accurate and repeatable delivery conditions through the use of a miniature pump and associated programmed control means. In many cases, the storage pressure is desirably less than ambient body pressure to prevent undesired leakage of the medication from the medication chamber into the body of the patient, and thereby positively prevent accidental overdose during certain failure modes. For one illustrative example of an implanted medication infusion pump of this general type, see U.S. Pat. No. 4,573,994.
In the past, the medication within the pump medication chamber has been subjected to the desired storage pressure by forming at least a portion of the medication chamber as a movable wall shared with a pressure reservoir charged with a selected pressure fluid. More particularly, the pressure fluid has been provided in a combined liquid-vapor state such as a selected fluorocarbon, wherein the pressure fluid undergoes liquid-vapor change of state at normal body temperature to appropriately expand or contract the pressure reservoir in a a manner acting through the movable wall to maintain the medication chamber under substantially constant pressure conditions. As the medication chamber is expanded upon filling with the medication, the pressure fluid undergoes significant state change to the liquid phase to reduce the volumetric size of the pressure reservoir. Conversely, as the medication is delivered in doses to the patient, the pressure fluid progressively undergoes state change to the vapor phase to reduce the volumetric size of the medication chamber and maintain the medication under substantially constant pressure. Freon 113 has been used to maintain the medication at a slight negative or subambient pressure in response to normal patient body temperature and altitudinal variations up to about 8,500 feet above sea level.
In many prior implantable infusion pumps, the movable wall separating the pressure reservoir from the medication chamber has been constructed in the form of metal foil diaphragm or bellows devices which are relatively costly to manufacture and/or have exhibited unacceptable or unreliable operating performance. More recently, an improved and significantly simplified pressure reservoir has been proposed in the form of a flexible sack or bag constructed from an elastomer material and charged with the selected pressure fluid. See, for example, U.S. Pat. No. 5,167,633.
A flexible elastomer pressure reservoir offers significant advantage in terms of reduced manufacturing cost and complexity, in addition to a reduced volumetric reservoir size which increases the medication-containing capacity of the infusion pump and/or reduces the overall size of the infusion pump. However, when the medication chamber is completely filled with liquid medication, the pressure fluid within the elastomer reservoir may encounter complete phase change to a liquid state within the pressure reservoir. When such complete phase change occurs, renucleation of the liquid pressure fluid to the vapor phase may not occur spontaneously, but instead may require significant additional input energy. In this regard, normal dispensing of the medication to the patient in small doses may not displace the movable wall of the reservoir with sufficient energy to insure pressure fluid renucleation to the vapor state. Accordingly, if the pressure fluid is allowed to change entirely to the liquid state, further phase change back to the vapor state might not occur during normal pump operation whereby the pressure reservoir may fail to maintain the medication under the desired constant pressure conditions.
There exists, therefore, a need for an improved pressure reservoir in an implantable medication infusion pump, particularly with respect to providing means for preventing complete pressure fluid phase change to the liquid state. The present invention fulfills this need and provides further related advantages.